Hot-rolled carbon steel strips are produced by hot-rolling at high temperatures. On the surface of the hot-rolled steel strip, a scale layer having a thickness of several microns to several tens of microns is formed.
FIG. 1 shows an example of a process for descaling the hot-rolled steel strip according to the prior art. As shown therein, the scale on the hot-rolled steel strip is cracked by a scale breaker 3, and then chemically removed mainly using high-temperature strong acid, such as hydrochloric acid or sulfuric acid, in a pickling unit including a pickling bath 4 and a rinsing bath 5.
In FIG. 1, reference numerals 1, 2 and 10 indicate a payoff reel, a welder, and a tension reel, respectively.
However, this method for removing scale using strong acid has various problems, including the increase in the length of equipment, caused by the use of a number of pickling baths and rinsing baths, deterioration in the working environment, caused by the generation of acidic vapor, the occurrence of environmental hazards by waste acid treatment, the increase in accompanying facilities due to acid recovery and acid-resistant equipment, the difference in descaling ability between steel materials, deterioration in the quality of steel strips remaining too long in the acid solution and rinsing tanks during the stopping of a production line, etc.
As an alternative to this method for chemically removing scale using the acid solution, an attempt has been made to physically remove scale from the steel sheet surface by spraying shot balls, grit or a slurry mixture of shot balls or grit with water onto the steel sheet surface. Techniques related thereto may include Japanese Patent Publication Nos. 2002-532633, 2002-275666 and 2002-371314.
However, these techniques are mostly used together with a pickling process that is a pretreatment process for increasing the efficiency at which special steels such as stainless steel or electric steel sheets are pickled. Particularly, the application of these techniques to general carbon steels can hardly be seen.
In addition, Japanese Patent Publication No. 2004-74178 discloses a method of removing scale by spraying high-pressure water, and U.S. Pat. No. 6,328,631 discloses a method of spraying ice grains. However, the descaling performance of these methods and whether these methods can be used in practice are not clear.
Also, Japanese Patent Laid-Open No. 2003-064461 and U.S. Pat. No. 6,088,895 disclose techniques related to equipment in which said physical descaling process is continuously connected with a rolling or plating process, and a method for treating steel sheets using said equipment.
Japanese Patent Laid-Open No. 2003-064461 merely proposes a tension leveler or a shot-blasting method as a subsidiary means for pickling, and does not specifically mention shot blasting.
U.S. Pat. No. 6,088,895 discloses a method comprising the steps of tension-leveling and shot-blasting a metal strip, subjecting the shot-blasted strip to two-step brushing to remove the scale, controlling the surface roughness of the strip within a predetermined range, and finely controlling the surface roughness to less than 0.3 microns in a subsequent cold-rolling process. The method disclosed therein aims to provide a cold-rolled stainless steel sheet having low surface roughness.
However, the scale on hot-rolled stainless steel sheets is dense and difficult to remove, and for this reason, it is difficult to remove the scale using only the method of said US patent. In fact, the method of said US patent is used in combination with an acid pickling process.